EVOLUTIONARY HISTORY OF NORTHERN-HEMISPHERE NUCELLA (GASTROPODA, MURICIDAE) - MOLECULAR, MORPHOLOGICAL, ECOLOGICAL, AND PALEONTOLOGICAL EVIDENCE

By combining data from a variety of sources we explore patterns of evo lution and speciation in Nucella, a widely studied genus of shallow-wa ter marine neogastropods. We present a hypothesis of phylogenetic rela tionships for all of the currently recognized species of northern hemi sphere Nucella, based on an analysis of 718 base pairs of nucleotide s equence from the mitochondrial cytochrome b gene. The order of appeara nce of species in the fossil record is congruent with this hypothesis. The topology of the inferred phylogeny of Nucella, coupled with ecolo gical, morphological, and fossil evidence, was used to address three m ain questions: (1) At what time and by which route was the North Atlan tic invaded from the North Pacific compared to prior studies of the tr ans-Arctic interchange? (2) Do patterns of molecular variation within species corroborate the importance of climatic cycles in driving speci ation in north temperate marine animals? (3) Was radiation in the dire ction of increased or decreased ecological specialization, body size, or Vulnerability to predation? Molecular evidence confirmed that the s ole North Atlantic species, N. lapillus, arose from a North Pacific an cestor. Biogeographic and paleontological evidence supported the dispe rsal of Nucella, and perhaps other interchange species, via the Eurasi an Arctic. Rather intriguingly, the linkage of N. lapillus to a wester n as opposed to eastern Pacific clade, and the biogeographic origins o f the eastern Pacific species, parallel closely similar patterns obser ved in another genus of rocky-shore gastropods, Littorina. This congru ence, in conjunction with information on the climatic and geographic h istories of the region, as well as the geographic arrangement of mtDNA haplotypes within Nucella species, supports a model of speciation in Nucella driven by cycles of climatic amelioration and deterioration th at began during the Miocene. Calibrations from the fossil record of Nu cella suggest that third position transitions and transversions accrue at a rate of 3-4% and 0.5% respectively per million yr. This supports an early participation by Nucella in the trans-Arctic interchange, as suggested by paleobiogeographic studies. Consistent with the unstable taxonomic history of species of Nucella, we found few nonmolecular tr aits to be phylogenetically informative. Among North Pacific species, more recently derived species (N. canaliculata and the N. emarginata c lade) were more ecologically specialized (narrower diet and habitat ra nge). Consistent with extensive intraspecific variation, shell traits were quite labile evolutionarily: neither overall size nor development of antipredatory traits exhibited consistent evolutionary trends over the history of the genus. Nurse eggs (unfertilized eggs consumed by d eveloping embryos) were an ancestral trait that was lost evolutionaril y in the two clades that also exhibited increased body size, suggestin g that these two life-history traits may be coupled. The reduced numbe r of chromosomes in N. lapillus is clearly a derived state and is cons istent with White's (1978) observations on chromosome evolution in oth er clades.